The present invention relates to semiconductor packaging and, more particularly, to a method of making a semiconductor package with improved standoff.
Semiconductor dies include integrated circuits formed in Silicon that are usually packaged before being connected to other electronic devices or circuits. Such packaging usually entails attaching the die to a lead frame or substrate and then encapsulating the die and electrical connections with a mold compound. There are many types of packages available, some with leads extending out of the sides of the mold compound and others with an array of pads or conductive balls on a bottom surface of the package.
It is desirable to decrease the size and profile of a semiconductor package. Thin devices are known as low profile packages. One known low profile package is called a QFN (Quad Flat No lead). In a QFN type package, leads of the package are exposed at the sides and/or bottom surface of the package, with the leads being generally flush with the mold compound. Thus, attaching a QFN device to a substrate or Printed Circuit Board (PCB) can be difficult due to its low stand-off height. The stand-off height refers to the distance between the PCB and the semiconductor device.
FIG. 1 shows a conventional QFN device 10 attached to a PCB 12. The QFN device 10 may be attached to the PCB 12 with a solder reflow process. In reflow soldering, the QFN device 10 is attached to the PCB 12 with a solder paste and then subjected to heat, such as via a reflow oven, which melts the solder, thereby connecting the device 10 to the PCB 12, more specifically, connecting exposed leads 14 of the device 10 to corresponding connection points on the PCB 12, at a solder joint 16. As is apparent in FIG. 1, there is a very low stand-off height between the device 10 and the PCB 12. A low stand-off height may increase the level of stress at the solder joints 16 due to the differences in the coefficients of thermal expansion (CTE) of the die and the PCB, which means some of the solder joints 16 may be very weak. Additionally, the low stand-off height makes inserting an underfill material (not shown) between the device 10 and PCB 12 more difficult because many underfill materials resist flowing between a substrate and device with a low stand-off height.
One way to increase the stand-off height to allow for stronger solder joints is to attach solder balls to the exposed leads on the bottom surface of the device, e.g., a ball grid array (BGA). However, having to form the BGA adds time and expense to the packaging process. Further, solder balls can sometimes be dislodged prior to the device being soldered to a PCB due to mishandling. Thus, it would be advantageous to be able to increase the stand-off height for QFN or similar type packages without increasing the cost of production.